In general, a thermally-sensitive recording medium is obtained by coating a coating liquid on a substrate such as paper, synthetic paper, film or plastic, wherein said coating liquid is prepared by grinding and dispersing respectively a colorless or pale-colored electron-donating leuco dye and an electron-accepting color-developing agent such as a phenolic compound into fine particles, then mixing them together, and adding a binder, filler, sensitizer, slipping agent and other additives. The obtained thermally-sensitive recording medium develops color by an instant chemical reaction by heating using a thermal head, a hot stamp, a thermal pen or by heating by laser light and a recorded image can be obtained. A thermally-sensitive recording medium is widely used in a terminal printer of a facsimile or computer, an automatic ticket vending machine or a recorder for a measuring instrument, and along with the diversification of uses, a high-level image stability and stability of blank part are becoming to be required for the thermally-sensitive recording medium.
However, since an electron-donating leuco dye and an electron-accepting color-developing agent contained in a thermally-sensitive recording layer can be easily dissolved in many kinds of solvents, the thermally-sensitive recording medium has a problem that blank part develops color easily when contacted with water ink, oil ink or an adhesive and a problem that, when a chemical such as a plasticizer is adhered to the developed recorded image, the image is discolored. For the purpose of dissolving these problems, a technique of forming a protecting layer which is mainly composed of a pigment and resin on a thermally-sensitive recording layer is disclosed in Patent Document 1 and Patent Document 2.
Further, the use of a thermally-sensitive recording medium is expanding to various kinds of tickets, receipts, labels, use for an auto talking machine of Bank, use for inspection of a gas or electric meter or a note for a betting-ticket for horse racing or cycle racing. Therefore, very severe characteristics, which are not required up to the present, are becoming to be required for a thermally-sensitive recording medium. In these uses, since an outdoor use is becoming more common, characteristics and properties of a protecting layer that can endure more severe atmospheres, such as rain or a very high humidity, direct sunshine or the inside of a car during the summer season, are becoming more necessary compared with conventional use.
As the components that compose a protecting layer, for example, water-soluble polymers such as polyvinyl alcohol or starch are used as main components. For the purpose of providing a water-resistance to these components, the technique of using a crosslinking agent such as glyoxal is disclosed in Patent Document 3 and Patent Document 4, but the effect is not yet sufficient. Especially, in Patent Document 4, carboxy-modified polyvinyl alcohol is used as a binder for a protecting layer, epichlorohydrin and glyoxal are used as a crosslinking agent, and this technique provides a water resistance to the carboxy-modified polyvinyl alcohol by carrying out a crosslinking reaction of epichlorohydrin with the carboxyl group of the carboxy-modified polyvinyl alcohol and by carrying out the crosslinking reacting of glyoxal with hydroxyl groups of the carboxy-modified polyvinyl alcohol. However, since the crosslinking reaction rate is too slow, water-resistance is not accomplished instantly and it is difficult to display a sufficient effect just after the coating and drying process. Accordingly, in a case of using epichlorohydrin as a crosslinking agent, a long heat treatment, that is, curing process, is necessary, and problems of background coloring or workability, namely, the product cannot be obtained immediately after production are caused. Further, the water-resistance under severe conditions is not sufficient because the crosslinked part is easily hydrolyzed.
Further, in Patent Document 5, a technique of providing water-resistance by using a hydrophobic resin emulsion, such as an acrylic emulsion, is disclosed, however, since the heat-resistance property of the acrylic emulsion is not sufficient, printing run-ability such as head debris or sticking is harmed, and there is a problem of workability, that is, a desired coating amount cannot be obtained because the viscosity at a high share is low.
Patent Document 1: JP S48-30437 A publication
Patent Document 2: JP S48-31958 A publication
Patent Document 3: JP H8-230324 A publication
Patent Document 9: JP H9-164763 A publication
Patent Document 5: JP H1-196389 A publication